Polyurethane dispersions comprising moieties derived from fatty acids having a number of carbon atoms of from 12 to 40 have already been described, in EP 1 026 186 A1. These polyurethane resins have been made by reacting drying or semi-drying oils with low molar mass hydroxy compounds having two or more hydroxyl groups and from two to twelve carbon atoms, and reacting this intermediate with high molar mass polyols having a number average molar mass of from 400 g/mol to 20 kg/mol, compounds that have at least two groups that are reactive towards isocyanate groups, and at least one group that may be an acid group or a group forming a cationic group after neutralisation, and polyfunctional isocyanate compound to form a prepolymer which is neutralised, dispersed in water and optionally reacted with a chain extender.
Similar polyurethane dispersion have been disclosed in EP 0 640 632 A1 where an isocyanate functional prepolymer is made from a diisocyanate, a dihydroxy alkanoic acid, and a drying oil which has been transesterified in a separate step with a polyfunctional alcohol to form a hydroxy-functional fatty acid ester, the prepolymer is then neutralised, and contacted with water and a diamine to form a chain-extended polyurethane-urea dispersion which can be crosslinked by air-drying.
In the case of acid-modified polyurethanes which are made water-dilutable by incorporation of anionic groups, the method of choice to introduce acid groups is co-reaction of acids that have at least two, preferably exactly two, further groups which may be hydroxyl, amino, or mercaptane groups that react with isocyanate groups to form urethane, urea, or thiourea linkages. Preferred are dihydroxyalkanoic acids such as dimethylol propionic acid or dimethylol butyric acid which have an acid group that is sufficiently inert towards the isocyanate groups due to steric effects to minimise decarboxylation and formation of amines by the reaction between carboxylic acid and isocyanate, which amine then usually reacts with a further isocyanate group to form a urea structure. As this reaction consumes a part of the isocyanate, there is a shift in stoichiometry which either leads to lower molar masses of the polyurethane formed, or which must be compensated by increasing the amount of polyisocyanate which may lead to unwanted chain branching by formation of allophanates. A further drawback of the use of dihydroxyalkanoic acids is their limited solubility which necessitates the addition of solvents such as N-methylpyrrolidone or acetone which have to be removed in a separate distillation step, or, when remaining in the dispersion, add to the VOC of paints based on these polyurethane dispersions as binders. Other acids having additional groups that react with isocyanate groups such as propoxylated 3-hydroxy-2-hydroxymethylpropane-1-sulphonic acid and 2-(2-aminoethylamino) ethane sulphonic acid are generally used in the form of their sodium salts, and have presently largely been replaced by the said dihydroxyalkanoic acids.
It is therefore an object of the invention to at least partly replace these said carboxylic or sulphonic acids that have two further groups that are reactive towards isocyanate groups, in order to suppress side reactions of the isocyanate, and also to do away with the need to add solvents to dissolve and thereby make reactive these acids.